An I_a mechanism was assigned for water exchange on the hexaaquaions Rh(OH₂)₆³⁺ and Ir(OH₂)₆³⁺ on the basis of negative ΔV_‡ experimental values (−4.2 and −5.7 cm³ mol^-1, respectively). The use of ΔV_‡ as a mechanistic criterion was open to debate primarily because ΔV_‡ could be affected by extension or compression of the nonparticipating ligand bond lengths on going to the transition state of an exchange process. In this paper, volume and energy profiles for two distinct water exchange mechanisms (D and I_a) have been computed using quantum chemical calculations which include hydration effects. The activation energy for Ir(OH₂)₆³⁺ is 32.2 kJ mol^-1 in favor of the I_a mechanism (127.9 kJ mol^-1), as opposed to a D pathway; the value for the I_a mechanism being close to ΔH_‡ and ΔG_‡ experimental values (130.5 kJ mol^-1 and 129.9 kJ mol^-1 at 298 K, respectively). Volumes of activation, computed using Connolly surfaces and for the I_a pathway (ΔV_‡calc = −3.9 and −3.5 cm³ mol^-1, respectively, for Rh³⁺ and Ir³⁺), are in agreement with the experimental values. Further, it is demonstrated for both mechanisms that the contribution to the volume of activation due to the changes in bond lengths between Ir(III) and the spectator water molecules is negligible: −1.8 for the D, and −0.9 cm³ mol^-1 for I_a mechanism. This finding clarifies the debate about the interpretation of ΔV_‡ and unequivocally confirms the occurrence of an I_a mechanism with retention of configuration and a small a character for both Rh(III) and Ir(III) hexaaquaions.

The adsorption of methanol on the (110) surface of γ-alumina was investigated using both ab initio and density functional theory quantum chemical methods. A [Al₃O₉H₁₀]⁺ cluster model comprising one tetrahedral and two octahedral aluminum cations were used to describe the surface and the mechanism of adsorption of methanol. This has allowed us to rationalize the stable structures of adsorbate and the mode of bonding. The IR frequency shifts between the gas phase and the adsorbed species were also calculated and they exhibit good agreement with experiment.